1. Field of the Invention
The present invention relates to an ultrasonic signal focusing method and apparatus for an ultrasonic imaging system, and more particularly, to an ultrasonic signal focusing method and apparatus for transmission-focusing and/or receipt-focusing of an ultrasonic signal emitted toward an object.
2. Description of the Related Art
In general, an ultrasonic imaging system uses an ultrasonic wave to show the internal sectional structure of an object such as a human body. The ultrasonic imaging system includes an ultrasonic signal focusing device and a imaging device. The ultrasonic signal focusing device emits an ultrasonic signal to an object, and converts the ultrasonic signal reflected from a discontinuous surface of acoustic impedance of the object, into an electrical signal, The imaging device uses the electrical signal to show the internal structure of the object.
In the ultrasonic imaging system, one of crucial factors required for functional improvement is an ultrasonic image resolution. It is steadily under development to improve the resolution. In case of the ultrasonic image, lateral resolution is worse than axial direction resolution. Focusing plays a major role of determining the lateral resolution. To improve the lateral resolution, the ultrasonic imaging system uses, as a probe, an array transducer comprised of a number of transducer elements, and performs transmission focusing and receipt focusing through an electrical signal processing. During performing receipt focusing, it is possible to use dynamic focusing in which the position of a focal point is successively moved from an position close to the array transducer to that far from it, considering the travelling speed of the ultrasonic wave. The dynamic focusing provides a better lateral resolution than the case where the focal point is fixed.
A general transmission focusing will be described below with reference to FIG. 1.
In FIG. 1, the x-axis is parallel with the ultrasonic transmission plane of an array transducer 10, and the z-axis is perpendicular to the ultrasonic transmission plane thereof. For clarity, a point where a scan line intersects the surface of a particular transducer element in the array transducer 10 is determined as the origin (0, 0), and the transducer element located at the origin is called a "transducer element 0." Here, the scan line connects the center of the ultrasonic transmission plane of the "transducer element 0" with a transmission focal point F in the case where a steering angle is .theta..
To make ultrasonic pulses emitted from all transducer elements reach the transmission focal point F at the same time, a transducer element farther from the origin should emit an ultrasonic pulse earlier than that closer to the origin do. For instance, a "transducer element 3" should transmit an ultrasonic pulse earlier by l3/v than the "transducer element 0" do. Here, "v" is the velocity of the ultrasonic pulse, "r" is the distance from the origin to the focal point F, and "l" is the distance difference between the distance r and the distance r+l from the ultrasonic transmission plane of the transducer element placed at the location which is not the origin to the focal point F. Thus, when the steering angle .theta. by which each of the scan lines indicated as solid lines rotated counterclockwise from the z-axis is used, a transmission delay time t.sub.d (x.sub.1) for a delay unit corresponding to a "transducer element 1" having a position x.sub.1 is obtained by the following equation (1). ##EQU1##
It is not possible to assign a negative transmission delay time value to the delay units. Therefore, in reality, it is required that all transmission delay time values become positive number by adding a positive value to the transmission delay time values corresponding to all transducer elements.
However, for convenience of explanation, it is assumed that it is possible to assign a negative transmission delay time value to the delay unit. Also, an instantaneous time when a "transducer element 0" transmits an ultrasonic pulse is defined as "0." Then, by replacing the r in the equation (1) by vt/2, transmission delay time or reception delay time corresponding to a transducer element having the center position x is expressed as the following equation (2). ##EQU2##
Here, "t" is time taken when the ultrasonic pulse reciprocates from a transducer element to the focal point F.
When a transmission delay time of the equation (2) is used for each electrical pulse generated in a pulse generator (not shown), the ultrasonic pulses transmitted toward the focal point F by the transducer elements form ripples as shown in FIG. 2. In FIG. 2, each ripple results from the fact that the transducer elements delay one pulse outgoing from the pulse generator by different transmission delay times and transmit the delayed result.
During transmission focusing, all ultrasonic pulses reach the focal point F at the same time, and all the ultrasonic pulses having reached the focal point F have the same phase. Thus, an amplitude or intensity of the ultrasonic wave becomes maximized at the focal point F. However, the ultrasonic pulses do not reach the location A or B different from the focal point F at the same time and have a different phase from each other. As a result, the ultrasonic pulses offset each other destructively and the intensity of the ultrasonic wave in the location A or B becomes smaller compared to the focal point F. In this case, as it is distant further toward the lateral direction than the axial direction, the intensity of the ultrasonic pulse becomes much smaller.
Resolution is determined as a combinational result of the transmission/reception focusing as described above. Since the focal point should be fixed during transmission focusing, the receipt focusing is ideal. Even in the case of the receipt focusing, the lateral resolution is best in the vicinity of the focal point and becomes comparatively poor in the different positions thereof.